1. Field of the Invention
A technical field of the present invention relates to a method for manufacturing a semiconductor substrate.
2. Description of the Related Art
In recent years, an SOI (silicon on insulator) substrate has been used for a semiconductor device intended to high-performance. By utilizing characteristics of a thin single crystal silicon layer formed over an insulating layer, transistors formed in an integrated circuit can be electrically separated from each other completely. Further, each transistor can be formed as a fully-depleted transistor, and thus a semiconductor integrated circuit with high added value such as high integration, high speed driving, and low power consumption can be realized.
As a method for manufacturing such an SOI substrate, a hydrogen-ion-implantation separation method in which hydrogen ion implantation and separation are combined is known. A typical process of a hydrogen-ion-implantation separation method is described below.
First, hydrogen ions are implanted into a silicon substrate to form an ion implantation layer at a predetermined depth from the substrate surface. Then, another silicon substrate which serves as a base substrate (supporting substrate) is oxidized to form a silicon oxide layer. Then, the silicon substrate into which hydrogen ions are implanted are disposed in close contact with the silicon oxide layer of the another silicon substrate which serves as a base substrate so as to bond the two silicon substrates. Then, by performing heat treatment, one of the silicon substrates is separated at the ion implantation layer to form a thin single crystal silicon layer.
The single crystal silicon layer formed in a manner like the above-described method usually has a thickness of approximately 50 nm to 300 nm, and is very thin. Therefore, the single crystal silicon layer formed in a manner like the above-described method is really suitable to be used for a transistor of which high integration, high speed driving, and low power consumption are required. On the other hand, in the case where use of a power device, a photoelectric conversion device, and the like is considered, the single crystal silicon layer is required to have a certain thickness from the viewpoint of improvement in withstand voltage, photoelectric conversion efficiency, and the like.
The thickness of a single crystal silicon layer formed by an ion-implantation separation method mainly depends on acceleration voltage when ion implantation is performed. The single crystal silicon layer becomes thin when acceleration voltage is made low because an ion implantation layer is formed at a small depth from the surface of the single crystal semiconductor substrate. On the other hand, the single crystal semiconductor layer becomes thick when acceleration voltage is made high.
This shows that acceleration voltage needs to be increased simply to make the single crystal semiconductor layer thick. However, in reality, it is not easy to form a thick single crystal semiconductor layer by increasing acceleration voltage. This is because in the case of using an ion implantation apparatus suitable for mass production (an apparatus capable of supplying a large amount of current), it is difficult to have acceleration voltage higher than a certain level due to its limitation. In the case of using an apparatus in which small current is supplied, acceleration voltage can be increased. However, it takes time to obtain a predetermined injection volume of ions, so that it is not preferable in terms of productivity. Further, in the case of accelerating ions with a high voltage exceeding 100 kv, there is a problem in a safety standpoint because harmful radiation may be generated.
In order to solve the above-mentioned problems, a method is examined in which a single crystal semiconductor layer is made thick by epitaxial growth (e.g., see Reference 1: Japanese Published Patent Application No. 2000-30995).